This invention relates to the field of speech amplifier circuits for battery powered radio receivers and more particularly to a circuit that reduces speech distortion and increases perceived loudness.
In the design of radio frequency receivers for the reception of voice transmissions, it is desirable to include a speech amplifier that can be adjusted to provide as much distortion free power output to the speaker as the listener may require. But, most amplifiers operate more efficiently near clipping and, for a given power output, an amplifier with a smaller maximum power output capability than another similarly designed amplifier will typically require less power input. Consequently, when the radio receiver is battery powered, it becomes desirable to limit maximum amplifier power capability to reduce battery drain and extend the time between battery charges or replacement. These two design goals are obviously in conflict and a compromise maximum power output capability is usually chosen for the speech amplifier when the radio receiver is battery powered.
In FIG. 1, a prior art radio receiver is illustrated. Referring to this figure, the demodulated speech output of a receiver "front end" 102 is coupled by a potentiometer 104 to a speech amplifier 106 and speaker 108. Potentiometer 104 functions as a volume control (a rotary control is assumed) and at some point in its rotation, the input signal to speech amplifier 106 will be sufficient to drive the amplifier into clip. This is graphically illustrated in FIGS. 2a, b and c wherein the thin and thick lines respectively represent the response of the prior art circuit of FIG. 1 and the response of the present invention of FIG. 4 (described below). In FIGS. 2a, b and c respectively, the gain of volume control 104, the perceived loudness to the listener, and the amplifier distortion are plotted against the rotation angle of the volume control.
As the volume control is advanced from its minimum volume position (the far left on the horizontal axis of the graphs) amplifier 106 begins to clip at rotation angle 202. FIG. 2a indicates that the amplifier input signal increases as the volume control is rotated beyond point 202 (to the right on the horizontal axis of the graphs); yet, FIGS. 2b and 2c respectively show that there is no substantial increase in the perceived volume and that the distortion increases rapidly as the volume control is rotated beyond point 202.
The amplifier clipping can best be understood by referring to FIG. 3 wherein a frequency response plot of a typical human voice is illustrated. The typical human voice has first, second and third peaks or "formants" 302, 304 and 306 centered approximately at 700, 1500 and 2400 Hz. The first formant can be as much as 15 dB stronger than the second, and the second formant can be as much as 6 dB stronger than the third.
When speech amplifier 106 begins to clip, frequencies within the band width of first formant 302 are distorted before those of second formant 304 and third formant 306. Unfortunately, additional distortion occurs because the second and third harmonic products of the first formant fall within the bandwidth of the second and third formants. Thus, distortion increases rapidly and there is no significant increase in the perceived loudness at the speaker once distortion begins. Accordingly, it would be desirable if the speech signal could be conditioned before amplification to reduce this distortion and increase the perceived loudness, while simultaneously maintaining intelligibility.